1. Field of the Invention
The present invention relates to an evaporation leak check system.
2. Description of Related Art
In recent years, emission control of fuel evaporation leaked from a fuel tank becomes severe. Especially US Environment Protection Agency (EPA) and California Air Resources Board (CARB) establish standard that requires detection of the evaporation leak from a minute opening in the fuel tank.
JP-B2-3896588 (US 2004/0000187) describes an evaporation leak check system. In the system, a reference pressure is defined by a pressure when air is made to flow through a reference orifice using a pump. Then, a pressure is detected when a fuel tank is decompressed or compressed by a pump, and the detected pressure is compared with the reference pressure. Thus, the system determines whether an evaporation leak amount from the fuel tank is within an allowable range or not.
In the system of JP-B2-3896588, air flowing through the reference orifice contains much foreign objects because the air is drawn from outside air, so that the reference orifice may be clogged by the foreign objects. If a diameter of the reference orifice is made smaller or if the reference orifice is completely closed by the foreign objects, the reference pressure is varied. In this case, the check of the evaporation leak may become inaccurate. If a filter that collects the foreign objects is arranged upstream of the reference orifice, the number of components included in the system is increased, and construction of the system may become complicated.
JP-A-2007-077950 (US 2007/0062495) describes an evaporation fuel handling apparatus. When a pump is driven in a state that a passage upstream or downstream of the pump is closed, a pressure in the passage is detected as a closed-state pressure. Further, a pressure difference is obtained between a pressure detected when air is made to flow through a reference orifice using a pump and a pressure detected when air containing evaporation fuel is made to flow through the reference orifice using the pump. A concentration of the evaporation fuel is calculated based on the pressure difference and the closed-state pressure. However, because it is necessary that each of the air or the air containing the evaporation fuel is made to flow through the reference orifice, the calculation of the concentration of the evaporation fuel may become inaccurate if the reference orifice is closed by foreign objects contained in the air or the air containing the evaporation fuel.